1. Field of the Invention
This invention relates generally to the field of control circuitry for appliances and more particularly to a control system for a clothes dryer.
2. Description of the Related Art
It has been common practice to provide automatic clothes dryers with an electromechanical timer which the user manipulates to select the desired dryer cycle. The electromechanical timer provides a means for the user to input desired control information and it also operates to switch on various machine loads. While the use of a electromechanical timer is a cost effective and familiar control device for operating a dryer, there are some disadvantages in a timer based control system. For example, when using a timer in a straight timed setting manner, the user typically must estimate, based on experience, the amount of time needed to dry a particular load. Such estimating can result in under or over drying.
In an effort to overcome the shortcomings of a straight time setting, many dryers are provided with a certain degree of automatic control based upon sensing load dryness. The typical approach utilizes a moisture sensor device in combination with an electromechanical dryer. The clothes dryer is operated with the timer de-activated until a preselected dryness condition is sensed at which point the timer, which is set by the user, is activated. The drying cycle is terminated when the timer times out. While combining a moisture sensor means with a timer does make the control more responsive to the clothes condition, these systems are relatively inaccurate, inflexible and often result in longer drying times than actually necessary.
Electronic controls offer an alternative to the traditional electromechanical timer based dryer controls and can be used to improve the dryer cycle responsiveness to the sensed moisture in a clothes load. For example, U.S. Pat. No. 3,762,064, to Offut, discloses a fully electronic dryer control system for a clothes dryer wherein the length of the dry cycle is responsive to the sensed dryness of the clothes. Clothes dryness is sensed by a pair of electrodes. To ensure complete drying, an add-on interval of time is added to the end of the sensed drying period. The duration of the "add-on" time is dependent on the length of the sensed drying interval and the dryness condition selected by the user at the initiation of the drying cycle.
U.S. Pat. No. 4,477,982, to Cotton, discloses a fully electronic, microprocessor based control system which senses the moisture content of clothes in a dryer drum via moisture sensing sensors or electrodes. The sensors are engagable with wet fabrics for completing an electrical current path therethrough wherein input signals are supplied to the microprocessor responsive to the completion of the electrical current path through the sensors. Counting apparatus is associated with the microprocessor for accumulating a count of the input signals. The microprocessor is operable for initiating termination of the fabric drying cycle when series of signals fail to accumulate to at least a predetermined number in a predetermined sensing time period.
While fully electronic systems offer some opportunities to enhance dryer performance and responsiveness to the sensed dryness condition, there are also some disadvantages. Specifically, electronic dryer control systems do not provide a ready way to communicate the status of the drying operation to the user unless a relatively expensive electronic display is provided. Moreover, dryer users are more familiar and comfortable with electromechanical timer type control systems for dryers.
U.S. Pat. No. 5,481,169, to Turetta et al., is an example of an effort to provide a microprocessor based appliance control system with the benefits of a traditional timer selector knob. In this reference, a stepping type motor is connected through a gear drive system to a selector knob. The selector knob is meant to provide an appearance and functionality similar to the conventional electromechanical timer knob. The position of the selector knob is communicated to a microprocessor via a potentiometer, an angular transducer or any known switch. In this manner the selector knob can be used to input data to the microprocessor and the microprocessor can energize the stepping type motor to rotate the selector knob and communicate cycle information to the user. While this control system offers some benefits, it still is relatively costly and does not combine an electromechanical timer having cam operated switches with a microprocessor. Moreover, this system does not provide a manner of operating a clothes dryer to minimize dry cycle length based on sensed dryness condition while providing feedback through the operation of a timer during the drying cycle.
Accordingly, it would be an improvement in the art to combine the cost effectiveness, familiarity and cycle progress feedback features of an electromechanical timer control system with the improved control sophistication and responsiveness of a microprocessor based control system. Moreover, it would be an improvement in the prior art to more accurately determine the length of dry time needed to adequately dry clothes in clothes dryer having a combined or hybrid electromechanical timer and microprocessor control.